Historically, turboprop engines have been controlled from the cockpit via two levers. A speed lever adjusts engine speed and a power lever adjusts engine torque. To control the engine, a pilot would adjust the speed lever to a desired engine speed setting. Then, the pilot would "close the loop" on torque by watching a gauge and moving the power lever until the desired torque was obtained.
More recently, however, digital engine controls have been employed to adjust engine torques. The digital engine controls issue torque commands, which are based upon ambient conditions including temperature, altitude and mach number, and maintain engine torque at commanded levels via closed loop control.
Torque mismatches can occur when digital engine controls are utilized to manage the power of an aircraft having multiple turboprop engines. Each engine receives torque commands from its own digital engine control, and each digital engine control has its own air data computer for reading ambient conditions. However, the air data computers do not always indicate the same ambient conditions. For example, the air data computer for one engine could read one temperature, while the air data computer for another engine reads a different temperature. Because torque is calculated as a function of data, the two engines would be commanded to different torque levels. The resulting mismatch in torques would cause undesirable yaw.